122 
PROF. W. E. DARBY: RESEARCHES ON THE ELASTIC 
§ 5. Looping after the Elastic Limit of Proportionality has been Passed. 
The recorder is fitted with a microscope so that the process of stretching can be 
watched as the experiment proceeds, and the loading, which is produced by hydraulic 
pressure, stopped at any moment. This arrangement enables interesting records to 
be taken, because after the test piece has been stretched an assigned amount, the load 
can be let off and then immediately re-applied, so that stretching continues through 
a second interval and so on. 
Such a record is seen in fig. 5. The material is nickel steel. It will be seen that 
25V Ouuy 1918. 
Fig.<£ (nickel steel). 
the removal and the re-application of the load compels the spot of light to trace a 
loop. The area of the loop represents the internal work done during the process. 
Following the path of the spot it starts from the origin O and describes the elastic 
line OA, passes the limit of proportionality at A, and then curves away to the yield 
point B, and on to C. At C the loading is stopped, the hydraulic pressure is relieved 
by opening the exhaust valve, and the spot travels down the curved path CD as the 
load falls to zero. The exhaust valve is then closed and the pressure valve is opened 
and the process is repeated through the path EF and so on. 
When the steel test piece has been stretched beyond its limit of proportionality, 
for example to C, fig. 5, the total extension is made up of two parts, namely :—- 
(1) the proportional elastic extension up to the limit of elasticity, for example up 
to A, fig. 5 ; 
(2) the plastic extension after the limit has been passed. 
